Hydrocracking is a process in which heavy oil fractions recovered from crude oil are combined with hydrogen gas and subjected to high temperatures and pressures in one or more reactors filled with catalyst. The catalyst helps the longer chain hydrocarbons in the heavy oil fractions break or “crack” into smaller hydrocarbon molecules that eventually form kerosene, naptha, and gasoil components.
The effluent from the hydrocracker reactor is typically fed to a distillation column or fractionator. The fractionator separates the feed stream into different fractions of liquid hydrocarbons each having a desired boiling range. Generally speaking, “lighter” hydrocarbons (e.g., liquid hydrocarbons having a lower boiling point) are withdrawn from the top and sides of the fractionator as an overhead and sidedraw streams. Heavier fractions (e.g., hydrocarbons having a higher boiling point) collect in the bottom of the fractionator and are known as bottoms. The bottoms are recycled to the hydrocracker reactor or undergo other processing.
The hydrocracker reactor and fractionator work together, in conjunction with other process equipment such as heaters, heat exchangers, phase separators, pumps, compressors, etc., and the operating conditions of one can be dependent on or tied to the other. For example, an operator or an automated control system monitors the yield of fractionator bottoms, which may be reflected by changes in the level of bottoms collected in the bottom section of the fractionator or by changes in the flow of fractionator bottoms effluent. If, in one instance, an operator of the hydrocracking unit sees an unexpected increase in the yield of fractionator bottoms, then the operator should seek to determine the cause of the increase. The increase in fractionator bottoms could be due to, for example, a reduction in conversion (“cracking”) in the hydrocracker, thus resulting in sending heavier feed to the fractionator. Or, the increase could be due to separation problems in the fractionator. If the increase is due to a reduction in hydrocracker conversion, then the usual adjustment is to increase the hydrocracker reaction bed temperature. If the increase is due to separation problems in the fractionator, then the usual adjustment is to increase the sidedraw flow rate.
A careful examination of the fractionator operation can provide an indication of the problem, but such an examination can sometimes be time consuming. Also, certain solutions such as making pressure corrections to the fractionator operation are difficult adjustments to make in the timely manner required for controlling a continuous production processes. The application of the method of controlling the fractionator by increasing its sidedraw product flow risks pulling too much liquid from the fractionator column which can cause off-specification product. And, increasing hydrocracker reaction temperature in order to control the fractionator can increase energy costs and result in “re-cracking” of hydrocarbons into undesired lighter fractions such as naptha.
Among these options, however, the control of the fractionator operation by adjusting the hydrocracker reaction bed temperature can at times be a preferred approach due to a lower risk of producing off-spec product. However, over time and after several temperature adjustments, “re-cracking” inefficiencies can increase to a point where there is significant economic loss due to over cracking.
Considering the above-noted problems, there is a need for an improved system and method for controlling the hydrocracker reactor and fractionator of a hydrocracking process and system. Such a system and method should provide an operator with a greater level of certainty in controlling unexpected increases in fractionator bottoms yield and provide a more efficient process control methodology for controlling the fractionator and hydrocracker reaction loop.